The present invention relates generally to pre-distortion of an input signal to a non-linear amplifier. In particular, it pertains to a pre-distorter for a non-linear amplifier that does not actually implement the kernels of a Volterra operator used to describe the non-linear amplifier nor the inverse kernel of the first kernel of the Volterra operator.
When designing amplifier, the desired goal is to obtain an amplifier without any distortion. This is practically impossible to do because some of the components, such as transistors, tubes, etc., used in an amplifier are non-linear. Furthermore, the capacitors and inductors used in an amplifier cause frequency dependent distortion. For an RF (i.e., radio frequency) amplifier, this results in two different effects. One effect is that the output signal is distorted at the carrier frequency and the other effect is that there are harmonics in the output signal at multiples of the carrier frequency. In order to deal with such distortion, the input signal can be pre-distorted (i.e., pre-shaped) with a pre-distorter before it is sent to the nonlinear amplifier.
To derive a corresponding pre-distorter for a non-linear amplifier, a standard approach is to describe the amplifier in terms of a Volterra operator . The advantage to doing so is that, given that the inverse kernel of the first kernel of the Volterra operator for the non-linear amplifier exists, the corresponding pre-distorter can be derived by describing it in terms of yet another Volterra operator. A good discussion of Volterra operators can be found in Schetzen, Martin, The Volterra and Wiener Theories Of Nonlinear Systems, John Wiley and Sons, New York, 1980, which is incorporated by reference herein.
The Volterra operator for a pre-distorter derived using this approach will be in terms of the kernels of the Volterra operator for the corresponding non-linear amplifier and the inverse kernel of the first kernel of the Volterra operator for the nonlinear amplifier. Unfortunately, this means that a pre-distorter will have to implement these kernels. As evidenced in The Volterra and Wiener Theories Of Nonlinear Systems, this is very messy and impractical since determining the kernels is in general awkward and, as the order increases, the practical implementation of the kernels becomes excessively complicated. Therefore, there is a need for an efficient method for deriving pre-distorters, particularly those with high orders, that do not actually implement these kernels.
In summary, the present invention comprises a method of deriving a pre-distorter for a corresponding non-linear amplifier. The present invention also comprises the pre-distorter itself.
The first step of the method is to define the pre-distorter""s topology. This is done by first determining a corresponding inverse topology to the non-linear amplifier""s topology. The non-linear amplifier has a topology comprising filter and non-linear amplifier elements. The inverse topology comprises corresponding inverse filter and amplifier elements to the amplifier""s filter and non-linear amplifier elements. Then, each of the filter and non-linear amplifier elements of the amplifier for which compensation with the pre-distorter is desired are identified. Any of the inverse filter and amplifier elements corresponding to one of the filter and non-linear amplifier elements that is not identified is then removed from the inverse topology. This forms the pre-distorter""s topology so that it comprises the corresponding inverse filter and amplifier elements to the identified filter and non-linear elements.
The next step of the method is to define the selected inverse filter and amplifier elements"" responses. To do so, an input signal is applied to the non-linear amplifier. In response, measurements of selected signals of the nonlinear amplifier are made. Once the measurements are made, coefficients that define the selected inverse filter and amplifier elements"" responses are estimated. This is done in response to the measurements that were made.